VHP Photo Glossary: Caldera

Photograph by M. Williams, National Park
Service, 1977.

Caldera

A caldera is a large, usually circular depression at the summit of
a volcano formed when magma is withdrawn or erupted from a
shallow underground magma reservoir. The removal of
large volumes of magma may result in loss of structural
support for the overlying rock, thereby leading to collapse
of the ground and formation of a large depression. Calderas are
different from craters, which are smaller, circular depressions
created primarily by explosive excavation of rock during eruptions.

Aniakchak Caldera, pictured here, formed during an enormous explosive eruption
that expelled more than 50 km3 of magma about 3,450 years ago.
The caldera is 10 km in diameter and 500-1,000 m deep. Subsequent
eruptions formed domes, cinder cones, and explosion pits on the
caldera floor.

More about calderas

Did you know?

At least 1,299 episodes of unrest have occurred at 138 calderas greater
than 5 km in diameter during historical time.

In a typical year some form of unrest (earthquakes, ground deformation,
change in fumarole activity, or eruptions) occurs at about 18 large
calderas worldwide, and eruptions occur within or near at least five of
them.

The largest eruption of the 20th century from the Novarupta vent
in the Valley of 10,000 Smokes of Alaska, ejected about 12
km3 of magma and resulted in the formation of a caldera 3
km across. Amazingly, the caldera collapse didn't occur at the eruption
vent, but 10 km away at Katmai, a stratovolcano! Apparently magma
drained away from Katmai's magma reservoir to Novarupta's erupting vent.

Yellowstone National Park consists of three enormous calderas that
erupted about 2, 1.2, and 0.6 million years ago. The most recent
caldera is 45 km across and 75 km long!

Caldera-forming eruptions are the largest eruptions on Earth. For
example, the Fish Canyon eruption in southwestern Colorado (United
States) about 28 million years ago erupted more than 5,000 km3
of magma from La Garita caldera. That's enough magma to bury
the entire state of California to a depth of nearly 12 m!